I know the first numbers appear to indicate capacity, but sometimes the number changes not because of different displacement but to indicate that it has a balancer. What do the letters mean, for example on a number like 156FMI?

This standard provides motorcycles and mopeds with engine (hereinafter referred to as engines) Model methodology.

2 reference standards

The following standards contain provisions, through reference in this standard constitute the provisions of this standard. Time of publication of this standard, as shown in both versions have
Efficiency. All of the standard will be amended to use the standards of the parties should explore the possibility of using the latest version of the following criteria possibilities.
GB / T 725-1991 Product name and model of the internal combustion engine preparation rules
GB / T 5375-1998 models of motorcycles and mopeds methodology

3 Definition of Terms

The criterion used by the following definitions
3.1 natural air-cooled engine
Use of the natural flow of the air-cooled engine cylinder and gas cylinder head, such as engine parts.
3.2 forced air-cooled engine
Fan generated by the mandatory use of mobile air-cooled engine cylinder and gas cylinder head, such as engine parts.

4 Model methodology

Engine Model by Arabic numerals (hereinafter referred to as figure) and the Chinese phonetic alphabet (hereinafter referred to letters) component. Engine Model followed including
The following four parts:

4.1 Department: Product features of the code for the enterprise or product line, code-named code-named, with more than three capital letters do not express, on behalf of one of product lines
No. subject to the industry focal point for Standardization approved the record.
4.2 Central: a few symbols from the cylinder, cylinder layout of the form of symbols, stroke symbols, bore symbols. Cylinder with one of several symbols that figure. Cylinder
Path symbols with cylinder diameter (mm) express integer. Symbols and layout of the form of cylinder-stroke symbols, respectively, according to Tables 1 and 2 requirements.

4.3 rear
The rear of the characteristics of symbols from the cooling mode, the engine uses the characteristics of symbols and the name of the engine displacement symbols. Cooling characteristics of symbols according to Table 3 Regulation
Set.

4.3.1 Characteristics of the engine uses symbols: fixed for the letter "M".
4.3.2 the name of the engine displacement symbols: According to GB / T 5375-1998 on behalf of the provisions of division of displacement, according to the provisions of Table 4 letters.
4.4 tail
Tail for the design and modified code. Design code to use Arabic numerals 1,2,3 ... ... followed by express order of engine design, when design, code-named 1
Can be omitted. Modified code to use capital letters A, B, C, D ... ... followed by express order of the original engine type modifications. Design and modified code to use symbols "-"
And in front of symbols separated.

Many thanks humanbeing. I'm amazed that something that useful can be deleted, but I've noticed that chinariders only keep threads for so long. Not a good idea, as a lot of knowledge and tips are lost that way.